Recent estimates indicate that -two-thirds of adults in the US are overweight or obese. Insulin resistance, a common feature associated with obesity, increases the risk of type 2 diabetes. The cellular mechanisms responsible for the development of diet-induced insulin resistance are complex and not fully understood. However, a key observation has been the reduced signaling through the IRS1 PI 3-kinase pathway and diminished glucose uptake and utilization in skeletal muscle. Two complementary mechanisms have emerged as potential mediators of the reduced IRS1 PI 3-kinase signaling: enhanced basal IRS-1 serine phosphorylation and altered expression of the PI 3-kinase p85 regulatory subunits. Recently, we observed that 3d of overfeeding in humans enhanced p85a expression and correlated with the reduced insulin sensitivity. We hypothesize that increased p85or in skeletal muscle is an early defect in the development of dietary-induced insulin resistance and accompanies the key changes in IRS-1 serine phosphorylation. Our studies will focus on identifying the mechanisms for diet-induced dysregulation of p85cr and the relationship between p85or and IRS-1 serine phosphorylation as mediators of skeletal muscle insulin resistance. [unreadable] [unreadable] [unreadable]